The invention relates to a substrate-coating system equipped with a substrate holder for holding at least one substrate at a coating position where it is coated on a coating side, a substrate heater, and a method for heating that at least one substrate during coating.
Substrate coating systems of that type are generally known and are used for, for example, depositing functional and/or protective coatings on optical components. Heating substrates during coating will frequently allow improving the properties of the coatings deposited. In the case of optical coatings, substrate temperatures of, e.g., about 300° C., are typical.
IR thermal radiators whose peak emission wavelengths range from 2 μm to 6 μm, depending up the type of thermal-radiation source employed, are usually employed for heating substrates. However, this type of thermal-radiation heating of substrates presents problems if the substrate material only weakly absorbs in that wavelength range, which is particularly true for calcium-fluoride (CaF2) substrates, and thus also for commonly employed optical components, such as lenses, fabricated from that material. Employing IR thermal radiators to heat such substrates is particularly ineffective and involves long heating periods if the thermal radiators are arranged at a relatively large distance from that side of the substrates that is to be coated in order not to shield that side of the substrates and thereby interfere with the transport of coating material from a coating source to that side of the substrates. Another problem that occurs in the case of CaF2-substrates is the comparatively large coefficient of thermal expansion of CaF2, which requires carrying out heating and cooling-down cycles sufficiently gently that cracking due to thermally induced mechanical stresses will be avoided.
Laid-open publication DE 100 45 264 A1 proposed employing thermal-radiation generators that emit at wavelengths longer than 9 μm for heating workpieces, particularly such consisting of CaF2-substrates. Specifically, employment of a CO2-laser having an emission wavelength of 10.6 μm was proposed.
Laid-open publication WO 00/00445 A1 describes a method for thermally conditioning glass substrates prior to a subsequent surface treatment and an associated, evacuatable, heating chamber. The thermal-conditioning process is the only process that takes place in that chamber. Substrates are brought to another, separate, chamber for the subsequent surface treatment, which might involve, e.g., vacuum coating the substrates. Within the heating chamber, substrates are mounted on a substrate holder and irradiated from one side with thermal radiation at wavelengths that preferably range from 1.5 μm to 6 μm emitted by one or more heat lamps. A reflector is arranged on the inner wall of the heating chamber, at least on that side thereof opposite the substrates, preferably on all sides thereof, at a distance from the substrates specifically in order to reflect thermal radiation transmitted through the substrates back to the substrates and thereby to provide a certain, passive, heating effect on the other sides of substrates, i.e., those opposite the heat lamp or lamps, which is regarded as preferable to an alternative where substrates are actively irradiated from both sides by suitable heat lamps. The heating chamber, which is specifically designed for thermally conditioning substrates, in particular, flat glass substrates, is not suited to simultaneously serving as the chamber to be employed for the subsequent surface treatment of substrates, such as coating substrates, due to its design.